Error Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

Question

I am trying to target .NET 4.6 and also take advantage of the latest C# version by changing the C# language version to 6.

However during compilation I got this error:

Error Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

If I update the /langversion:6 in Web.Config setting to 5 it works,

  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701">

But how do I resolve this without resorting to lower language version?

I am using Visual Studio 2015 Community Edition, and I have also installed .NET Framework 4.6 manually just in case it was not installed by default by Visual Studio 2015.

The project is a standard ASP.NET MVC template project created by Visual Studio 2015.

@modosansreves That will revert to default language version.. which is C# 5 I believe... please check whether you can use any of C# 6 language feature. — Rosdi Kasim, Dec 07 '15 at 10:34

score 92 · Accepted Answer · edited Aug 11 '18 at 11:56

92

Pay attention to compiler "type" in the Web.Config file, when changing Framework version:

for 4.5 and C#5 -

type="Microsoft.CSharp.CSharpCodeProvider...

for 4.6 and C#6 -

type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"

More info in this forum post

edited Aug 11 '18 at 11:56

glennsl

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answered Aug 14 '15 at 10:35

DrewB

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This works!.. Although I consider it as a bug in Visual Studio 2015 for not updating this automatically when I re-target my application to .NET 4.6 and C# 6. – Rosdi Kasim Aug 14 '15 at 11:41
I think you're right in that. I got stung with it and stumbled across your question. Luckily the next link I found was the answer. – DrewB Aug 15 '15 at 04:38
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Is there universal CSharpCodeProvider for all .NET targets? – denfromufa Sep 11 '17 at 16:19
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My web.config had correct type and language version was set to default `` Just cleaning the solution and recompiling fixed it for me in VS 2019. – joym8 Mar 30 '20 at 19:30

score 37 · Answer 2 · edited Jun 13 '18 at 13:41

37

Update the following NuGet packages (whichever installed) to resolve the problem:

Microsoft.CodeDom.Providers.DotNetCompilerPlatform
Microsoft.Net.Compilers

edited Jun 13 '18 at 13:41

Neo

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answered Feb 02 '17 at 19:57

Developer

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this is more of a comment than an answer – Al Lelopath Feb 02 '17 at 20:15
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This is the real solution!! Uninstall this nuget package and problem solved for me – Geomorillo Mar 02 '17 at 20:08
I switched to c# 6.0, but the package was still on an older version. Updating from 1.0 to 1.0.4 worked for me. – Mixxiphoid May 13 '17 at 23:48
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@AlLelopath just because it's a small answer doesn't make it a comment. This tiny "comment" solved the issue for me and for others, so it better stay as an answer. – Shimmy Weitzhandler Nov 22 '17 at 05:56
For those who needs: "update-package Microsoft.CodeDom.Providers.DotNetCompilerPlatform" and then "update-package Microsoft.Net.Compilers" – Theotonio Jan 02 '21 at 00:19

Prashant vishwakarma · Answer 3 · 2016-11-09T23:25:54.787

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1.Go to Project and select your Project properties.

2 Select Build and Click Advanced Button.

3.Select Language Version default.

4.Save .

https://youtu.be/IP8feQeWqkk

edited Nov 09 '16 at 23:25

answered Nov 09 '16 at 23:10

Prashant vishwakarma

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Had this error in VS2013. This solution didn't work for me unfortunately. – Jviaches Jul 17 '17 at 20:30
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This one worked for me. All the other suggested solutions revolved around changing the web.config file, or alternatively, downloading NuGet packages / updates. The compiler kept throwing errors about an invalid version, which confused me since I hadn't had that setting in the config file. Turns out the advanced build properties wasn't automatically updated by the NuGet package update. – Kiran Ramaswamy Nov 11 '17 at 22:33

score 4 · Answer 4 · answered Jun 28 '20 at 09:43

4

Open NuGet Package Manager console and run this following command

Update-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform -r

answered Jun 28 '20 at 09:43

Mahfuj Ur Rahman

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this is great but why? – AtLeastTheresToast Mar 01 '21 at 21:17

score 3 · Answer 5 · answered Jun 28 '16 at 11:46

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I met such behavior if project was not fully upgraded to VS2015. In case if you open solution from 2015 in VS2013 - you will get this error

answered Jun 28 '16 at 11:46

Sergii Lischuk

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Me too, I think this error basically means you need to install VS2015! Someone else on the project is using newer language features that 2013 can't cope with – JonnyRaa Oct 06 '17 at 11:46

score 1 · Answer 6 · answered Jun 02 '20 at 14:43

1

I had similar issue when I was loading application for the first time in visual studio 2019. Updating "Microsoft.CodeDom.Providers.DotNetCompilerPlatform" nuget to later version did the trick for me.

answered Jun 02 '20 at 14:43

Govind

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score 0 · Answer 7 · edited May 23 '17 at 12:26

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See an answer to ASP.NET strange compilation error!.

Downgrading your C# language to 5 only masks the problem, as soon as someone uses a C# 6 feature your website/project will break and you will still spend time fixing it again. Try the solution in the link.

edited May 23 '17 at 12:26

Community

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answered Apr 07 '16 at 21:28

PBo

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Error Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

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